1) Field of the Invention
The present invention relates to a support sheet for a photographic printing sheet. More particularly, the present invention relates to a support sheet for a photographic printing sheet having an enhanced resistance to fogging and yellowing and thus capable of being printed with clear photographic images.
2) Description of the Related Arts
Formerly, a baryta paper sheet was used as a support for a photographic printing sheet. The baryta paper sheet was produced by coating two surfaces of a paper sheet having a good sizing property and mechanical property with a coating material containing a white pigment, for example, barium sulfate.
Recently, a waterproof paper sheet composed of a substrate paper sheet and coating layers formed on two surfaces of the substrate sheet and comprising a polyolefin resin have become widely used as a support sheet for a photographic printing sheet, in place of the baryta sheet.
The photographic printing sheet comprising the waterproof support sheet is advantageous in that, in a developing step for the photographic printing sheet, the highly hydrophobic polyolefin coating layer obstructs a penetration of a developing solution into the support. sheet, and accordingly, the time needed for washing and drying the printing sheet can be shortened, and the shrinkage and elongation of the support sheet be restricted, and thus the photographic printing sheet exhibits a superior dimensional stability.
Nevertheless, the polyolefin resin-coated support sheet is disadvantageous in the following items.
The polyolefin resin coating layer contains an inorganic white pigment, for example, titanium dioxide, for enhancing an opacifying power and a resolving power of the resultant photographic printing sheet but this pigment has a poor dispersion in the polyolefin resin. Also, the pigment contains a volatile substance, and in a melt-extruding step of the polyolefin resin, the volatile substance forms bubbles in the polyolefin resin melt, and thus the resultant polyolefin resin coating layer is sometimes cracked.
To avoid the above-mentioned disadvantages, the amount of the white pigment to be added to the polyolefin resin cannot be increased to a high level sufficient to obtain a satisfactory opacifying and resolving power of the resultant photographic printing sheet. Generally speaking, when the white pigment consists of titanium dioxide, it is difficult to add the titanium dioxide pigment in an amount of about 20% by weight or more to the polyolefin resin. Accordingly, the photographic printing sheet prepared from nhe conventional polyolefin resin-coated support sheet does not have a satisfactory sharpness of the images printed thereon.
Recently, a support sheet for a photographic printing sheet having an electron beam-cured resin coating layer formed by coating a surface of a substrate paper sheet with an electron beam-curable resin composition comprising an organic unsaturated compound curable by an electron beam irradiation, and irradiating an electron beam to the coated layer of the resin composition, was disclosed in, for example, Japanese Examined Patent Publication Nos. 60-17,104 and 60-17,105 and Japanese Unexamined Patent Publication No. 57-49,946.
In this type of support sheet, the resin composition to be coated on a surface of a substrate paper sheet need not be heated at a high temperature, and thus can contain the inorganic white pigment in a large amount of 20 to 80% by weight. Therefore, the resultant photographic printing sheet produced from this type of support sheet can record thereon photographic images with a significantly enhanced sharpness, in comparison with those of the conventional polyolefin resin-coated photographic printing sheet.
Nevertheless, this type of photographic printing sheet, in which a photo-sensitive layer is formed on an electron beam-cured resin coating layer, is disadvantageous in that, when developed with a developing solution of photographic chemicals, a portion of the developing chemicals is adsorbed by and remains on the electron beam-cured resin coating layer, and causes the printing sheet to turn yellow after the development. Also, when developed after storage for a long time, non-neglectable fogging occurs in the developed photographic printing sheet, or the photo-sensitivity of the photographic printing sheet is changed.
Various attempts have been made to eliminate the above-mentioned disadvantages. For example, Japanese Examined Patent Publication No. 1-21,495 discloses an attempt to form a polyethylene coating layer on an electron beam-cured resin coating layer, to thereby restrict the change in the photosensitivity during a storage of the photographic printing sheet. This attempt is disadvantageous, however, in that, to obtain a satisfactory prevention of the change in the photosensitivity, the polyolefin coating layer must be formed in a large thickness, and this causes the sharpness of the resultant photographic images to become unsatisfactory, even though the electron beam-cured resin coating layer is employed to increase the sharpness of the printed images.
Japanese Unexamined Patent Publication No. 60-144,736 discloses an attempt to arrange a barrier layer between a substrate paper sheet and an electron beam-cured resin coating layer, to thus restrict any change in the photographic sensitivity of the photographic printing sheet. The barrier layer made from the materials disclosed in the Japanese Publication, however, is not satisfactory when trying to prevent the occurrence of fogging after storage for a long time.
Also, Japanese Unexamined Patent Publication Nos. 62-61,049 and 61-141,543 discloses a specific polymer or monomer for forming the barrier layer, but this specific polymer or monomer does not satisfactorily remove the above-mentioned disadvantages.
Further, Japanese Unexamined Patent Publication No. 59-124,336 discloses a barrier layer arranged between a substrate paper sheet and an electron beam-cured resin coating layer and prepared from at least one member selected from aqueous solutions of water-soluble polymeric material and dispersions of polyolefin homopolymers and copolymers and polyacrylate and polymethacrylate homopolymers and copolymer, to restrict the change in photographic sensitivity.
The barrier layer made from the polymeric material disclosed in the Japanese Publication does not provide a satisfactory prevention of fogging of the resultant photographic printing sheet after a storage thereof for a long time.
Generally, it is known that the relationship between the energy level of the electron beam applied to an electron beam-curable compound composition and the fog density of the resultant photographic printing sheet due to a developing solution is contrary to the relationship between the energy level of the electron beam and the yellowing density. Namely, when the electron beam is applied in a large energy level the yellowing caused by the developing solution is restricted to a low level but the fogging is promoted to a high intensity. Also, when the electron beam is applied in a low energy level, the yellowing density is significantly increased, whereas the fog density is decreased, and the physical properties, for example, adhesive strength and mechanical strength, of the resultant cured resin coating layer are poor.
Accordingly, to prevent or restrict the yellowing and fogging of the photographic printing sheet without affecting the physical properties of the cured resin coating layer, it is necessary to control the energy level of the electron beam to an optimum level. Also, to eliminate all of the above-mentioned disadvantages, it is very important to provide a new type of support sheet capable of preventing the yellowing and fogging of the resultant photographic printing sheet without depending on the quantity of the electron beam applied in the formation of the crated resin coating layer.